This invention relates to the molding of articles from plastic preforms, and in particular it relates to a new and improved apparatus for transferring heated parisons from the oven to the molding means such as a blow mold or the like.
Certain molding techniques such as vacuum molding, blow molding or the like start with a preform of moldable material such as plastic or the like, which preform is commonly referred to as a parison, wherein during the molding procedure the preform is reshaped into a finished article on or within the mold. Such preforms can take many different shapes such as a disc or a tube. While the present invention is adaptable for use with any type of parison, it is particularly concerned with tubular parisons, and hence the invention will be specifically described with respect to its use in connection with tubular parisons.
Tubular parisons are formed and heated to a temperature suitable for molding such as blow molding in two different ways. According to one procedure, a parison is extruded in a hot state and while still hot it is carried over to the blow mold. In accordance with another procedure, the extruded parisons are cooled and/or stored for use at a subsequent time and/or place. This latter procedure is referred to as the "cold parison" procedure because it starts with a cold (i.e. room temperature) parison which must be heated to a temperature suitable for blow molding. The present invention is concerned with either of these methods since in either method the problem arises of transferring a heated parison to the mold. However, the transfer problem is somewhat greater in the cold parison procedure and hence the present invention will be specifically described with respect to this procedure. A suitable method and apparatus for carrying out the cold parison technique is described in detail in the assignee's previous U.S. Pat. No. 3,765,813, issued OCT. 16, 1973, and the disclosure thereof is incorporated by reference herein for details of a typical cold parison technique. In the cold parison apparatus, as described in said patent, the parisons are heated within an oven, in which parisons are supported in an upright position on carrier pins as they travel through the oven. The oven is designed and operated such that as the parisons reach the suitable forming temperature they arrive at a discharge station at which a transfer means having some type of gripping structure enters the oven from above through an opening, grips a properly heated parison, removes the parison upwardly out of the oven and then transfers the parison laterally to the blow mold whereat the parison is lowered. In the type of operation with which the said previous patent is concerned, as the parison is lowered, its lower end is gripped by a clamp located beneath the level of the mold while the transfer means still grips the upper end of the mold. The transfer means is then raised to thereby stretch the parison for biaxial orientation thereof. Hence, the transfer means including its gripper means has a multiple function of gripping the parison at the oven and removing therefrom, carrying the parison to the blow mold, lowering the parison such that its lower end is properly positioned with respect to its clamp and then effecting stretching of the parison. In contrast to the rotating arm transfer means as described in the said patent, there has been developed a transfer means comprising a carriage mounted on a horizontal rod for linear movement between the oven and the mold, the rod itself being movable vertically along upright frame means to permit raising and lowering of the carriage containing the gripper means at the oven and at the mold. In these arrangements the carriage further includes a fluid operated cylinder for opening and closing the gripping fingers of the gripper means. Transfer means of this type are shown in commonly owned copending U.S. applications Ser. Nos. 496,309, filed Aug. 9, 1974 and now abandoned in favor of Ser. No. 667,459 and 509,150, filed Sept. 25, 1974 and now U.S. Pat. No. 3,981,673.
However, even with the improved transfer means as described in the above mentioned pending applications, certain problems remain. Given a necessary relative positioning of the oven and the blow mold, the distance from the discharge station of the oven to the blow mold must necessarily be relatively great, i.e. great enough that the time required to transfer the parison between these two points is long enough that the parison cools down during this transfer time by an amount which adversely affects the precise heating profile of the parison. Also, the power means provided for effecting this transfer over this relatively long distance tends to be somewhat complex. On the other hand, however, these problems are not solved by simply providing a larger and faster power means since this would tend to increase the acceleration and deceleration forces exerted on the parison. This could cause a tubular parison to oscillate like a pendulum thereby causing deformation of the parison during the transfer and also preventing or at least inhibiting proper placement of the lower end of the parison onto the lower clamp at the mold.
Thus, there exists a need for improving the means for transferring parisons which overcome the disadvantages now existing in the art.